The present invention relates to antennas for the transmission and reception of microwave energy. More particularly, the present invention relates to an improvement to a microwave antenna for reducing undesirable echo.
In the field of space communications, a microwave antenna is used to transmit and receive many communications channels. One such antenna is the Cassegrainian antenna, which has a large concave main reflector, a smaller convex subreflector placed forward of the main reflector and a feed horn located centrally in an opening in the main reflector. Radiation from the feed horn is reflected from the subreflector to the main reflector and is transmitted from the antenna as a narrow microwave beam.
Unfortunately, some radiation transmitted from the feed horn is also undesirably reflected back toward the feed horn from the subreflector. The feed horn and adjacent main reflector reradiate part of this energy in the original forward direction. This undesirable doubly reflected energy is called an echo. The echo causes an objectionable intermodulation background noise component in the communications channels which sharply increases as the antenna size and number of channels is increased. See Bell Telephone Laboratories, Transmission Systems for Communications, 4th Ed., pp. 517-522, 1970.
Heretofore, undesirable echoes have been reduced by placing an essentially flat reflecting plate near the subreflector between the subreflector and the feed horn to cancel most of the energy reflected back toward the feed horn. When the plate reflects radiation which is equal in amplitude and 180 degrees out of phase at a given frequency with the reflection from the subreflector, good echo cancellation at that frequency is obtained. For a small number of communications channels, an acceptably small echo can be achieved over a moderate bandwidth. However, as the number of channels is increased, the echo at the edges of the band must be sharply reduced. An acceptable level cannot be achieved with a flat plate. Furthermore, some communications systems use distinct frequency ranges for simultaneous transmission and reception. Consequently, as the number of channels is increased to take full economic advantage of the antenna, the echo-caused noise in the frequency ranges rises above an acceptable level if just a flat plate is employed.
Accordingly, it is an object of the present invention to substantially cancel microwave echo reflections over a wide bandwidth in a microwave antenna accommodating a large number of communications channels.
It is another object of the present invention to substantially eliminate echo-caused channel noise from a Cassegrainian antenna accommodating a large number of communications channels.
It is another object of the present invention to substantially eliminate undesirable echo interference to transmitted and received communications channels carried in distinct frequency ranges in a microwave antenna.